4/17 Billing date previous balance $1,100
4/27 Payment 700
4/29 Charge 300
5/7 Payment 60
Day action balance # of days with that balance
4/17 1,100 from 4/17 to 4/26 = 10 days
4/27 payment 700 400 from 4/26 to 4/28 = 2 days
4/29
charge 300 100 from 4/29 to 4/7 = 8 days
5/7 charge 60 from 5/7 to 5/16 = 9 days
5/16 none - -
Weighted average: [1,100*10 + 400*2 + 100*8 + 60*9] /
29 = 13140 / 29 = 435.10
Answer: 435.10